Power factor correction (pfc) circuits are typically used within power supply applications requiring ac/dc rectification. Rectifying arrangements for use in such an application may essentially comprise a full wave voltage rectifier, typically in the form of a diode bridge, and an output capacitor, to provide regulation of the output wave form. Such a rectifying arrangement, however, only draws current from the ac supply (typically a mains electricity supply) when the full wave rectified voltage is greater than the voltage across the output capacitor. This gives rise to an inefficient current profile of the input ac current consisting of separated narrow pulses of current having large peak values. This current profile has a high harmonic content and gives rise to a very low power factor (about 0.5-0.6) of the rectifying arrangement as a whole. The high harmonic content in turn gives rise to large line noise (typically reflected back onto the ac mains supply). Such a current profile, in addition to being inefficient, is also unacceptable under EC standard IEC1000-3-2.
A known solution to mitigate the problem of the inefficient current profile is to employ a pfc circuit which is inserted between the output capacitor and the diode bridge, and which essentially consists of an inductor followed by a diode with a switch connected between ground and between the inductor and the diode. By rapidly switching the switch on and off, current may be drawn from the input, full-wave rectified signal even when the input voltage is less than the output voltage. In fact, with such an arrangement the output voltage is always greater than the input voltage and for this reason such an arrangement is typically referred to as being in a boost configuration. Generally, the inductor, diode and switch arrangement may be termed a switched inductor pfc circuit and the circuit which controls the on and off times of the switch may be termed a pfc controller circuit.
With such an arrangement, each section of time when the switch is switched on to connect the inductor to ground may be termed an on time phase having an on time duration, t.sub.ON, with a corresponding off time phase having an off time duration, t.sub.OFF, occurring between each on time phase. A single cycle, having a period, .tau., comprises an on time phase together with an off time phase, the period .tau. being given by .tau.=t.sub.ON +t.sub.OFF. Each cycle will repeat with a frequency given by f=1/.tau. and the resultant on-off or pulse width modulated signal will have a duty cycle of t.sub.ON /.tau..
A known pfc circuit of this nature operates by comparing the output voltage of the rectifying arrangement with a fixed reference voltage and controlling t.sub.ON to maintain the output voltage at a fixed level. This generally results in a large difference between the peak value of the input voltage (typically from the mains electricity supply) and the substantially regulated output voltage. This large difference between the peak value of the input voltage and the output voltage requires a correspondingly large inductor. Furthermore, for a given output power requirement the duty cycle of the switch arrangement must also be correspondingly large for a large difference between the input and output voltages.
Furthermore, it is known to operate a power factor correction circuit in what is termed a critical discontinuous mode in which t.sub.OFF is such that the switch is switched back on again as soon as the current flowing through the inductor falls to substantially zero. In order to do this, conventional controllers measure the voltage across an additional winding which is coupled to the switched inductor, which voltage switches when the current flow through the inductor falls to zero. However, the provision of the additional winding represents an expense which adds to the total cost of the power factor correction circuit.
The present invention seeks to provide a power factor correction controller circuit which at least mitigates one or more of the disadvantages of a conventional pfc circuit as discussed above.